Retinal drug delivery remains a formidable challenge. Simple delivery methods results in poor drug distribution. Only 5% of topically applied drug enters the eye with <1% reaching the posterior segment.1 The blood retinal barrier also limits intravenous drug delivery to the retina. Therefore, intravitreal, subconjunctival or subretinal injections are the most efficient delivery methods.2 Current treatments for many retinal diseases involving pathological angiogenesis require the monthly intravitreal injection of a highly concentrated solution of Avastin® (bevacizumab), which may lead to systemic side effects.3 There is a great need to develop systems that dramatically decrease injection frequency to increase patient compliance, lower health care costs, decrease ocular complications (e.g., acute onset endophtahlmitis, pseudo-endophthalmitis, cataract progression, retinal detachment and hemorrhage),4,5 and minimize the risk of infections and systemic side effects,6 which will in turn allow us to deliver a larger selection of drugs. Hydrogels are an ideal vehicle for intravitreal drug delivery since they are injectable, biocompatible and hold large drug payloads.7 Even though hydrogels limit potential systemic side effects, their use clinically has been limited since they only marginally decrease injection frequency (every 2-3 months).3,8,9 
Protein and antibody therapeutics are becoming an increasing prevalent class of drugs due to selectivity, potency, and limited side effects compared to small molecule drugs.10 However, their short half-life and susceptibility to enzymatic degradation makes administration and delivery challenging. Often, invasive procedures are required to delivery proteins to specific locations. As such, drug loaded biomaterials have been proposed as protein delivery vehicles. Furthermore, many protein drugs require repeated and frequent doses due to their short half-life.
An emerging strategy for the sustained release of protein is through reversible affinity interactions. Affinity is often used to describe the favourable noncovalent interaction between two macromolecules, often involving electrostatic, Van der Waals, or hydrophobic interactions between two binding partners.11 As many proteins have inherent affinities with extracellular matrix proteins (e.g., basic fibroblast growth factor), affinity controlled release through mimicking the extracellular matrix is an appealing strategy.12 This method attenuates the initial burst release seen in many other delivery vehicles. The reversible affinity of many ligands can be exploited in order to create a system where release is dictated by the binding and unbinding kinetics as well as Fickian diffusion kinetics.13 